IP Multimedia (IPMM) is an example of a service that provides a dynamic combination of voice, video, messaging, data, etc, within the same session. By growing the numbers of basic applications and the media which it is possible to combine, the number of services offered to the end users will grow, and the interpersonal communication experience will be enriched. This will lead to a new generation of personalised, rich multimedia communication services, e.g. peer-to-peer multimedia communication, IPTV etc.
These services can be based on the IP Multimedia Subsystem (IMS) architecture, which is the technology defined by the Third Generation Partnership Project (3GPP) to provide IP Multimedia services over mobile communication networks (3GPP TS 22.228, TS 23.228, TS 24.229, TS 29.228, TS 29.229, TS 29.328 and TS 29.329 Releases 5 to 7).
The IMS makes use of the Session Initiation Protocol (SIP) to set up and control calls or sessions between user terminals (or user terminals and application servers). The Session Description Protocol (SDP), carried by SIP signalling, is used to describe and negotiate the media components of the session. Other multimedia applications which can be used for media transmission and control include Real-time Transport Protocol and Real-time Transport Control Protocol (RTP/RTCP), Message Session Relay Protocol (MSRP), and Hyper Text Transfer Protocol (HTTP).
FIG. 1 illustrates schematically how the IMS fits into the mobile network architecture in the case of a 3GPP PS access domain.
Call Session Control Functions (CSCFs) operate as SIP proxies within the IMS. The 3GPP architecture defines three types of CSCFs: the Proxy CSCF (P-CSCF) which is the first point of contact within the IMS for a SIP terminal; the Serving CSCF (S-CSCF) which provides services to the user that the user is subscribed to; and the Interrogating CSCF (I-CSCF) whose role is to identify the correct S-CSCF and to forward to that S-CSCF a request received from a SIP terminal via a P-CSCF.
A fundamental requirement for real-time service provision is the seamless handover of services for subscribers moving across cell boundaries of the radio access network (RAN) or subscribers moving across RAN or radio access technology (RAT) boundaries (e.g. moving between 2G, 3G and/or LTE networks). [The term ‘access domain’ will be used throughout to refer generally to any access network or access technology that the subscriber is moving across]. Traditional circuit switched (CS) based call services have been designed to meet this requirement. In the case of 2G and currently implemented 3G networks, packet switched (PS) real time handover (HO) with low latency is not provided for, although service continuity is achieved at the terminal side by ordering a session to be moved from one cell to the other, i.e. there is no prepare phase to shorten latency when moving between cells. Real time PS handover is standardized in 3GPP for 3G networks, but the feature has not yet been deployed. It is expected that when High-Speed Downlink Packet Access (HSDPA) is deployed, or shortly thereafter, the mechanisms needed for fast PS handover will also be deployed. For 2G networks, fast and efficient PS handover procedures in the packet switched (PS) domain have only recently been standardized in 3GPP TS 43.129, and in the initial implementation stage, roll-out of this feature across 2G networks will inevitably be patchy. In addition, support for PS handover in 2G networks such as GSM/GPRS networks is never likely to be comprehensive (if implemented at all), yet handover of PS calls would be desirable as 2G networks will continue to provide a fallback network for 3G subscribers in the case of limited 3G network coverage. It can also be expected that the next generation radio and core networks which are currently being specified under the names LTE (Long Term Evolution) and SAE (System Architecture Evolution) in 3GPP will also have limited coverage, and that these networks will also require fallback to 3G and 2G networks.
It is expected that in the future all peer-to-peer multimedia communications in the mobile network will run over the PS domain. In particular, as voice calls are just a special variant of peer-to-peer multimedia communication, all voice calls will be Voice-over-IP (VoIP) calls. VoIP calls will be particularly sensitive to even relatively minor service interruptions caused by inter-cell handovers. As long as a terminal engaged in a VoIP call can perform PS handover to another cell (the “target cell”), the interruption can be kept short enough to avoid any noticeable drop in perceived quality. However, if either the current cell (the “source cell”) or the target cell do not support PS handover, a noticeable interruption is likely to occur as packets will be lost or delayed during the transition period. Consequently, until all network technologies and all RAN cells support PS handover, the provision of IMS services such as voice and video calls utilising the PS domain is likely to result in users receiving a reduced quality of service when crossing cell boundaries.
International patent application number PCT/EP04/053333 describes a process for allowing the IMS to automatically establish a call over a CS network when a user requests the IMS call using signalling sent to the IMS over a PS network. In this way, the IMS ensures that the appropriate Quality of Service (QoS) is applied to the call. In addition, the IMS call using CS network will benefit from the seamless handovers afforded by the CS domain, if and when the user is handed over to a neighbouring cell. The procedure involves the establishment of a first CS leg between the terminal and the CS core network (terminating at a responsible Mobile Switching Centre (MSC)), and a second CS leg between the IMS and the MSC in order to link the CS access domain leg to the IMS. This is achieved using an inter MSC handover procedure, with the responsible entity within the IMS acting as an “anchor” MSC.
A potential disadvantage associated with the above proposal is that a VoIP call is automatically carried over the CS network even if the PS network is able to provide roaming and a satisfactory QoS for the call. In addition, the process of transferring the call to the CS network will consume significant resources within the network and, where the PS network can support the call, this will be wasted effort.
A possible solution to this problem is to include in system information messages, sent on a broadcast channel of the radio network, an indication of the level of support within the network for PS handover. This indication can be used by a wireless terminal to decide how to attach to a currently visited access network. However, for this to work efficiently, all networks must support the broadcasting of the appropriate system information. [Of course, where the PS access domain is operated by the user's home network, the IMS will have knowledge of the handover capabilities of the access domain.]